Moon

Abstract

The nearest extraterrestrial body to Earth is the Moon (only 356 410 to 406 697 km distant center-to-center, or about 30 Earth diameters). Earth’s Moon, with its diameter of 3476 km (27% that of the Earth’s diameter), is rather unique in that it has an unusually large ratio of satellite mass to planet mass, 0.12. Its mean density is much lower than that of Earth, 3.34 versus 5.52 g/cm³. The higher density on Earth results from the large amount of iron and nickel concentrated in its core. Siderophile elements appear to be deficient in the Moon, volatile elements also are depleted, but the refractory elements are much richer than on Earth. This distribution indicates that the Moon was very hot during the past and its volatile elements were vaporized and escaped into space. Five models have been proposed for the origin of the Moon. They are: (1) fission from Earth, (2) collision ejection due to impact of a Mars-size planetesimal with Earth, (3) disintegration by tidal forces of a planetesimal that entered Earth’s Roche Limit; the resulting debris formed an orbit around the Earth and served as the source for accumulation of the Moon, (4) coaccretion from planetesimals, and (5) capture (Brush 1986; Wood 1986).